Asthma is the result of host and environment interactions. Lower respiratory tract infections (LRTI), caused by viruses such as respiratory syncytial virus (RSV) and rhinovirus (RV), are a leading cause of bronchiolitis in infants. Infants hospitalized with bronchiolitis not only experience significant morbidity, but are also at significantly increased risk for both recurrent wheezing and childhood asthma. We have established that respiratory syncytial viral infections appear to directly contribute to asthma causation, not just simply identify persons at risk for subsequent wheezing. Viral infections outside the infant period are also important, as they are modifiable environmental factors that have been established to be the most common cause of asthma exacerbations in both children and adults. This proposal uses a combined, parallel clinical and experimental approach to evaluate the contribution of, causality, and mechanisms through which respiratory syncytial virus, and human rhinovirus, contribute to asthma development and disease natural history. These studies all relate to the central hypothesis that viruses, as one significant environmental factor, alter the risk for developing asthma, as well as the natural history of prevalent disease. Specific testable questions related to this hypothesis include: 1) to determine the host factors among those with RSV and HRV infection that contribute to asthma development, 2) to determine the timing of infection during infancy and how that impacts the risk of developing childhood asthma; 3) to determine the mechanisms through which RSV and HRV contribute to asthma development, specifically focusing on lung injury and differential immune response to infection; 4) to determine if there are allelic variations in host genes involved with severity of respiratory viral infection that correlate with a predisposition or resistance to both infant bronchiolitis and childhood asthma; 5) to determine if there are allelic variation in RSV genes that are associated with both infant LRTI severity, and subsequent risk of asthma development.